1. Field of the Invention
The field of the invention is that of synthetic and three-dimensional cartographic representation. The latter can be represented in head-up mode, as a conformal view, or in head-down mode. The expression conformal representation is intended to mean a cartographic representation which is perfectly superimposed on the terrain actually seen by the observer. Optical superposition is ensured by an optical system which projects the synthetic image towards the observer's eye. This optical system comprises an optical mixer or combiner which ensures the superposition of the image on the outside landscape. This type of representation is particularly used in aeronautics so as to ensure piloting under poor visibility or at night. In head-down mode, the synthetic view does not conform to the outside landscape (by definition) but can be positioned perfectly with respect to the primary piloting symbology displayed, if there is symbology.
2. Description of the Prior Art
A cartographic representation system is represented in FIG. 1. This system comprises a database representative of the terrain overflown, means making it possible to determine the position and the orientation of the viewing system displaying the cartographic image, an image generator CE and the viewing system D. The image generator CE ensures three main functions which are the computation of the viewpoint, the selection of the terrain zone to be displayed and the computation of the image to be displayed as a function of the viewpoint.
One of the tricky points with this three-dimensional representation is that it must be sufficiently complete and precise to afford a good representation of the terrain overflown which may be useful for piloting and for navigation and sufficiently discreet as not to saturate the natural image of the landscape.
Various solutions have been proposed.
A first solution for representing a 3D terrain compatible with a head-up viewing system is described in U.S. Pat. No. 8,264,498 entitled “System, apparatus, and method for presenting a monochrome image of terrain on a head-up display unit”. In this representation, the luminous intensity of the terrain is modulated as a function of the lighting parameters obtained by positioning a virtual pointlike or omnidirectional light source above the terrain overflown. This representation greatly overloads the display and does not correctly address the need for superimposition of the synthetic image with the real view.
A second solution consists in displaying the terrain with the aid of a simple grid as represented in FIG. 2. Distance perception is effected solely through the size of the geometric elements of the grid which are represented as white lines in FIG. 2. Indeed, under the effect of perspective, the greater their distance with respect to the displayed viewpoint, the smaller the geometric elements on the screen. This type of display is not suitable for the display of terrain very remote from the observer. U.S. Pat. No. 7,098,913 entitled “Method and system for providing depth cues by attenuating distant displayed terrain” proposes a solution for improving the representation consisting in darkening the terrain remote from the viewpoint. This technique allows better apprehension of the distance of the terrain but this does not solve the problem of saturation of the image.
A third solution which is a variant of the previous solution consists in modifying the size of the geometric elements onwards of a certain distance. For example, only one point out of two of the elevation mesh is retained. By way of example, FIG. 3 illustrates this type of graphical representation. One indeed succeeds in limiting the number of elements displayed on the screen, but one deletes a large part of the elements making it possible to improve the perception of the depth of the terrain. In fact, the representation of a 3D synthetic terrain by a grid is very suitable for the terrain close to the viewpoint.
A fourth solution consists in presenting the terrain by highlighting the ridge lines. The expression ridge line is intended to mean the boundaries between the hidden parts and the visible parts of the landscape. The ridge lines do not necessarily represent a series of points whose altitude is locally maximal. They are therefore dependent not only on the terrain but also on the position of the observer. The ridge lines must therefore be recomputed continually as a function of the position of the carrier. This makes it possible to afford some information about the relief of the terrain without overloading the image. Such a representation is depicted in FIG. 4. This representation of the terrain by the ridge lines makes it possible to greatly limit the information density displayed on the screen. It affords partial vision of the environment, this being beneficial for the perception of the remote terrain. However, it affords only extremely restricted information on the terrain close to the viewpoint.
Two of the graphical representations cited hereinabove can be used simultaneously. Thus, it is possible to represent the close terrain in the form of a grid and the remote terrain in the form of ridge lines as is seen in FIG. 5. Between the representation of the terrain in the form of a grid and the representation in the form of ridge lines, the transition is ensured by a principle of crossfading based on the terrain distance. Up to a distance close to the previously defined distance, the terrain is represented in both modes. Then, as a predefined distance is approached, the plot in the form of a grid becomes toned down while the ridge line representation is accentuated. The latter solution makes it possible to actually decrease the information on the screen and seems suitable for displaying the terrain information situated in the background but engenders artefacts of display. These artefacts are, for example, the sudden appearance and/or disappearance of the ridge lines when we are close to the viewpoint, the absence of potentially relevant terrain information such as valleys or thalwegs. Consequently, the latter representation does not yet provide an optimal level of terrain perception.